Soak fish in cool water for several hours. In a heavy skillet, heat oil to a moderate temperature. Drain fish and pat dry, then fry gently in the oil for 5 or 6 minutes. Add onions, parsley, scallions and tomatoes, and stir frequently until onions are soft and transparent. Then reduce heat, add garlic and ginger, and simmer for 15-20 minutes or until fish is cooked and flavors have blended. Serve with rice.